High molecular weight vinyl aromatic polymers, particularly polymers having weight average molecular weights (Mw) of greater than 300,000, have been typically produced by anionic polymerization rather than by free radical polymerization due to the slow polymerization rates used in free radical techniques to achieve high molecular weight polymers. However, anionic polymerization processes require expensive anionic initiators, such as organolithium compounds, and tend to produce discolored products due to the presence of residual lithium-containing salts.
High molecular weight vinyl aromatic polymers have also been produced by free radical polymerization in the presence of a soluble organic acid having pKa of 0.5 to 2.5, as in U.S. Pat. Nos. 5,145,924 and 5,115,055. However, in these processes the acid does not bind to the polymer and can migrate from the polymer during use, which can cause corrosion of mold surfaces. Additionally, such strong acids are difficult to disperse in vinyl aromatic monomers without initiating cationic polymerization and producing low molecular weight fractions.
Copending application 08/606,182 by Priddy, et al., discloses a process for preparing a high molecular weight polymer from a vinyl aromatic monomer using free radical polymerization, characterized in that the polymerization is conducted in the presence of a vinyl acid having a pKa at 25.degree. C. from about 0.1 to about 2.0. The vinyl acid catalyzes the polymerization reaction such that high molecular weight polymers are produced in reasonable reaction times and the C--C double bonds of the vinyl acid react with the vinyl aromatic monomer such that the vinyl acid is not free to migrate from the polymer. However, such vinyl acids are also difficult to disperse in vinyl aromatic monomers.
Therefore, there remains a need for an acid catalyzed free radical polymerization process for producing high molecular weight polymers in reasonable reaction times from vinyl aromatic monomers using free radical polymerization without the disadvantages of the prior art.